Capacitor electrodes

ABSTRACT

An electrode includes a current collector that is two-dimensional in shape and that contains perforations, and an intermediate layer that is electrically conductive and substantially corrosion-resistant. The intermediate layer is on at least part of a first surface of the current collector, and includes at least one of precious metal, graphitic carbon, a metal nitride, and a metal carbide. A first electrode layer is on the intermediate layer. At least part of the first electrode layer is in at least some perforations of the current collector. The first electrode layer, the intermediate layer, and the current collector are bonded together.

BACKGROUND

The electrodes of some electric components, such as pseudo-capacitorsand electrochemical double-layer capacitors, contain, as electrodematerial, activated carbon, metal oxides, such as ruthenium, nickel andmanganese oxide, or conductive polymers, such as polythiophenes,polyanilines, or polypyrroles. This electrode material is frequentlyapplied in powder form to current collectors with favorable electricalconductivity characteristics or produced on the current collectors viachemical or electrochemical deposition processes. The electricallyconductive current collectors are frequently present in the form of thinmetal foils, such as aluminum foils. In the case of aluminum foils, thealuminum oxide on the surface of the foil, which increases electricresistance, is removed by etching, for example, so that the electricallyconductive intermediate layer and then the electrode material can besubsequently applied. The surface of the aluminum foil is frequentlyenhanced by producing a surface topography via etching, for example.This enhances the surface of the electrodes, thereby resulting incapacitors with higher capacitance.

To improve the bond between the current collector and the electrodematerial, an electrically conductive intermediate layer, such asgraphitic carbon, is frequently applied to the current collector. Tosome extent, the particles of the intermediate layer can also be mixedinto the electrode material.

Especially in the case of electrochemical double-layer capacitors forhigh-performance applications, the current collectors and the electrodematerials arranged thereon are produced as thinly as possible, so that alarge area of electrodes connected in parallel can be incorporated intoa predetermined capacitor volume. As a result, the volume-specificbonding resistance between the current collector and the electrode layeris reduced, leading to more powerful capacitors.

As a result of stress caused by electrical operation or temperaturechange, the contact between the electrode coating on the electrodematerial and the current collector deteriorates. This leads to anincrease in the serial resistance of the capacitor, and thus to higherresistive losses during the operation of these capacitors. This increasein serial resistance is caused by corrosive effects on the surface ofthe current collector and separation of the electrically conductiveintermediate layer from the surface of the current collector.

SUMMARY

It is, therefore, a goal of the present invention to provide anelectrode that does not demonstrate an increase in serial resistanceduring the operation of the electrical component.

This goal is achieved with an electrode according to claim 1.Advantageous embodiments of the electrode, as well as method of itsmanufacture, are the subject of further claims.

An electrode according to the invention includes a two-dimensionallyshaped, electrically conductive current collector, which containsperforations. Each perforation passes through two opposing main surfacesof the current collector, thereby forming a continuous hole in thecurrent collector. On at least one main surface of the current collectorand in these perforations, a first electrode layer is arranged which isform fitting with the current collector and tightly bonded to thecurrent collector. The electrode layer can cover the perforations and bearranged in their perforations, wherein the edges of the perforationsbetween the opposing main surfaces can also be covered by the electrodelayer. Also present is an electrically conductive, corrosion-resistantintermediate layer, which is form-fittingly arranged between the currentcollector and the electrode layer, wherein the intermediate layercomprises materials that are selected from: precious metals, graphiticcarbon, metal nitrides and metal carbides.

The electrically conductive, corrosion-resistant intermediate layer,which is form-fittingly arranged between the current collector and theelectrode layers, serves to improve the bond between the currentcollector and the first and, if applicable, second electrode layer. Theintermediate layer is then arranged on the current collector and in itsperforations, wherein the electrode layers are then arranged on theintermediate layer.

It is also possible that the intermediate layer is completely arrangedon the current collector; that is, it completely covers the main surfaceof the current collector. In this case, an especially strong bondbetween the current collector and the electrode layers is possible.

Because of the perforations in the current collector and the positivefit, the first electrode layer can be applied to the current collectorin a well-adhering manner. In this process, the perforations result inan especially tight “interlocking” between the current collector and theelectrode layer arranged thereon. Because of this close contact betweenthe current collector and the first electrode layer, electrodesaccording to the invention demonstrate particular stability duringelectrical operation, in capacitors, for example. In electrodesaccording to the invention, the above-mentioned corrosion effects andthe separation of the intermediate layer from the surface of the currentcollector do not occur to the degree to which they occur in conventionalelectrodes.

In an advantageous embodiment of the electrode according to theinvention, a second electrode layer is arranged on the other mainsurface of the current collector. In this advantageous embodiment of theelectrode layer according to the invention, the first and secondelectrode layers, which are arranged on the two opposing main surfacesof the current collector, can bond to one another in the perforations ofthe current collector, resulting in especially strong adhesion of theelectrode layers to the current collector. In this variant of theelectrode according to the invention, a separation of the two electrodelayers and consequently the above-mentioned corrosion effects cantherefore be prevented especially effectively during electricaloperation. In this arrangement, it is possible for the first and secondelectrode layers to be made of the same material.

In the case of the series connection of electrodes, such as thecapacitors in a capacitor battery, it can be advantageous to use firstand second electrode layers made of different materials. In thisarrangement, it is possible, and especially advantageous, to produceindividual capacitors in a capacitor battery, the capacitorsdemonstrating different electrical properties as a result of theirdifferent electrode layers.

The current collector can comprise a punched and elongated aluminumfoil. In this arrangement, it is possible that the perforations, e.g.,in the form of holes or slits, are punched in the current collector,which is then elongated by stretch-forming it. Stretch forming, in thisarrangement, also increases, advantageously, the two-dimensionalexpansion of the current collector. If the current collector consists ofa metal foil, the perforations, in a current collector according to theinvention, can comprise between 25 and 70 percent of the surface area ofthe projection surface of the metal foil.

It is also possible that the current collector comprises a net of metalwires, which, for example, is woven from aluminum, nickel or rust-freestainless steel wires. In this case, the mesh in the net represents theperforations. At the same time, the perforations increase the contactsurface between the collector and the electrode layer, resulting inespecially powerful capacitors with low series resistance values. Theclose bond between the current collector and the intermediate layer,which may be present, and the electrode layers, especially in theperforations, simultaneously increases the long-term stability of theelectrodes.

Foam metals can also be used as current collectors with perforations.These metals are foamed and made porous via gas formation in the melt.The gases form bubbles in the metal, and these bubbles form hollowspaces in the metal foam once the metal has cooled and hardened. Thewalls of these hollow spaces are very thin in comparison to the areas ofthe foam metal in which there are no hollow spaces. By selectivelyetching these areas of the foam metal, e.g., with acids or bases, thehollow spaces can be opened on both sides, so that continuous openings,or perforations, can be produced. Aluminum and/or nickel, for example,can be used as the metal.

The intermediate layer can comprise precious metals, such as gold andsilver, as well as inorganic, corrosion-resistant materials, such ascarbon, in the form of graphite, for example, as well as metal nitridesand carbides, such as TiN, TiC, tungsten nitrides, and tungstencarbides. Electrically conductive polymers can also be used. It is alsopossible to mix the material used as the intermediate layer into theactual electrode material, making it possible to achieve an especiallystrong bond.

The first and/or second electrode layer can comprise conductivepolymers, such as polythiophenes, polyanilines, or polypyrroles, as wellas activated carbon. These materials allow for a tight interlockingbetween the current collector and the electrode layers, so that anintermediate layer is not needed to improve the bond. It is alsopossible that the first and/or second electrode layer comprises metaloxides, such as the previously mentioned ruthenium, nickel or manganeseoxides.

An electrochemical double-layer capacitor is also an object of theinvention. It comprises the electrodes of the invention, which arearranged opposite one another, wherein a porous separator is arrangedbetween the electrodes. The separator and the electrodes are impregnatedwith an electrolyte. In this arrangement, carbon, in the form ofactivated carbon or graphite, is used as an electrode material, forexample.

Electrodes according to the invention are also advantageous in hybridcapacitors, in which case a first electrode according to the inventioncomprises carbon and is arranged opposite a second electrode accordingto the invention, which comprises metal oxides and/or electricallyconductive polymers. A porous separator is again arranged between theelectrodes, the electrodes and the separator being impregnated with anelectrolyte. Electrodes according to the invention can also be used inthe pseudo-capacitors that comprise two electrodes, which eithercomprise electrically conductive polymers alone or metal oxides.

It is advantageous to use porous polymer films, non-wovens, felts orwoven materials made of polymers or fiberglass, as well as paper, asseparators.

Because of the electrodes according to the invention, both theelectrochemical double-layer capacitors and the pseudo-capacitorsdemonstrate improved permanent series resistance during operation.

A method of producing an electrode according to the invention consists,in its most general form, of two process steps. In a process step A), atwo-dimensionally shaped current collector containing perforations isproduced. Then, in a subsequent process step B1), an electricallyconductive, corrosion-resistant intermediate layer is produced on thecurrent collector, this intermediate layer comprising materials selectedfrom among:—precious metals, graphitic carbon, metal nitrides and metalcarbides. Then, in a process step B), an electrode layer is produced onat least one main surface of the current collector in such a manner thatit is form-fittingly and tightly bonded to the current collector.

In an advantageous embodiment of the method according to the invention,the electrode layer can also be produced on both main surfaces of thecurrent collector in process step B).

To produce the current collector in process step A), it is possible topunch continuous holes into a metal foil, for example, and then tofurther treat the metal foil by stretch-forming it. During stretchforming, the metal foil flows primarily out of its thickness, whereinthe surface of the metal foil becomes enlarged in comparison to itscondition prior to stretch forming. A person skilled in the art isfamiliar with the process of stretch forming, which is generallyperformed by placing the metal foil into a clamping tool and thenstretching it. When subjected to a considerable stretching force, themetal foil begins to flow as its thickness decreases. It is alsopossible to slit the metal foil and then subject it to stretch forming,so that the continuous holes (perforations) are only formed during thestretch forming. The thickness of the stretched metal foil generallyranges between 20 and 100 micrometers, wherein approximately 25 to 70percent of the surface area of the originally untreated metal foil isremoved by the production of the perforations, resulting in theformation of the continuous holes.

The continuous holes in the metal foil can also be formed viacontactless processes, e.g., by a laser burning the holes into the metalfoil. Subsequently, the surface of the metal foil can be enlarged bymeans of the stretch forming described above.

It is also possible to produce the current collector by weaving metalwires into a metal net. In this process, the continuous holes are formedby the spaces in the metal net. It is advantageous to use aluminum,nickel or rust-free stainless wires.

If an aluminum foil is used as the metal foil for the current collector,it is advantageous, prior to process steps B1) and B), to remove surfacelayers of the foil in a separate process step A1) to improve theconductivity of the current collector. Aluminum foil often contains apoorly conductive aluminum oxide surface layer, which can be strippedoff. This can be achieved by chemical, galvanic process steps, or byplasma etching, all of which are known to a person skilled in the art.

In process step B1), the electrically conductive intermediate layer, inthe form of a metal layer for example, can be produced via a galvanicprocess or by chemical vapor deposition (CVD) or physical vapordeposition (PVD). In chemical vapor deposition, metals or carbon areoften deposited out of the gas phase. Physical vapor deposition isgenerally accomplished by applying ionized particles in an electricfield. It is also possible, in process step B1), to produce a carbonlayer comprised, for example, of graphitic carbon as an electricallyconductive intermediate layer, by means of dip coating in a carbon bath.To this end, the current collector is placed into a dipping bathcontaining graphitic carbon with aqueous or organic solvents, such asalcohols, which can then be vaporized. If the intermediate layer isexecuted in the form of a metal layer, it is advantageous to use, inparticular, corrosion-inhibiting metals, e.g., noble metals such as goldor silver.

In process step B), the electrode layer can then be produced via bladecoating with a liquid or viscous phase containing an electrode material.It is also possible to first apply the liquid or viscous phasecontaining the electrode material to a carrier foil, such as apolytetrafluoroethylene foil, and then to dry it. The electrode materialcan then be transferred from the carrier foil to the current collector,thereby forming the electrode layer. This makes it possible, forexample, to transfer the electrode material located on the carrier foilby laminating it onto the current collector. During this laminationprocess, the electrode layer to be applied becomes especially tightlyand form-fittingly bonded to the current collector as well as itsperforations. If electrode layers are produced on both main surfaces ofthe current collector, they can become bonded together in theperforations, as mentioned earlier, so that the current collector isenclosed in an especially well-adhering manner. The carrier foil doesnot melt during application of the electrode layer. Instead, it can bepeeled off after the electrode layer has been transferred to the currentcollector. It is advantageous to apply the electrode material to thecarrier foil mixed with a binder. In this case, the transfer of theelectrode material to the current collector is achieved by melting thebinder. Polypropylene powder, for example, can be used as the binder.

In the following, the invention will be explained in greater detail,using exemplary embodiments and figures.

DESCRIPTION OF THE DRAWINGS

FIGS. 1 and 2 show the production of the current collector with theperforations, in process step A).

FIG. 3 shows the application of the electrode paste by means of reverseroller coating, in process step B).

FIG. 4 shows the application of the electrode paste by means oflamination, using a carrier foil.

FIG. 5 shows a complete electrode with an enlarged section of the lip.

DETAILED DESCRIPTION

FIG. 1 shows how, in process step A), perforations are produced in thecurrent collector 1, which is depicted here as a metal foil. Theseperforations in the metal foil can be produced in the form of continuousholes 5A via a punching tool 2, or perforations can be produced in theform of slits 5B. For space-saving reasons, the two alternatives formsof the perforations are shown in one figure.

FIG. 2 shows how, in process step A), the current collector 1, in theform of a metal foil, is stretched by means of stretch-forming followingproduction of the perforations 5. To this end, the current collector canbe clamped into a stretch-forming press 3, comprised of tongs 3A and astretching table 3B, for example. The stretched current collector can beobtained by pulling apart the tongs, although it is also possible toexpand the slits 5B by means of stretch forming. At the same time, thestretch forming also increases the surface area of the current collectorin an advantageous manner. The arrows schematically depict the directionof stretch forming.

FIG. 3 shows process step B) of the method according to the invention,in which the electrode paste 25, which can be present in a liquid orviscous phase, is transferred to the current collector 1 by reverseroller coating. During reverse roller coating, the electrode material 25is uniformly transferred to the roller 20B, using a blade 21 on theroller 20A. This roller then uniformly transfers the electrode materialonto the current collector 1. It is also possible to transfer theelectrode material to the current collector via a blade coating process.In this arrangement, it is possible to cover only one main surface ofthe current collector with the electrode material or, as shown in thefigure, both main surfaces.

FIG. 4 shows how the electrode layer 15 can be produced on the currentcollector 1 via an alternative process, lamination, using a carrier foil30. To this end, the electrode material 25 is first applied to thecarrier foil 30 and then dried. Then the carrier foil is brought intocontact with the current collector in such a way that the electrodematerial 25 arranged thereon bonds to the current collector directly.Then the electrode material can be transferred from the carrier foil 30to the current collector by rolling hot laminating rolls 22 across thecarrier foil, so that the electrode material becomes tightly bonded tothe current collector. In this process, the perforations 5 in thecurrent collector provide for an especially intimate and form-fittingapplication of the electrode material. If, as shown in FIG. 4, electrodelayers are produced on both main surfaces of the current collector, theycan come into direct contact with one another in the perforations 5 ofthe current collector, thereby enclosing said current collectorespecially tightly and form-fittingly. After lamination, the thenseparated carrier foil 30A can be pulled off. For bonding theelectrodes, it is possible to leave an area 4 of the current collectorfree of electrode material, so that electrical terminals can later beconnected in the completed electrical component by means of this bondingarea 4.

FIG. 5 shows a completed electrode with perforations 5 and the electrodelayer 15. An enlarged segment of a lip 6 shows that parts of the currentcollector that are not covered by the electrode layer are still visibleon the lip. If the current collector is a net of metal wires, it ispossible, for example, that individual wires 1A protrude from theelectrode layer and/or that, in the case of current collectorsconsisting of nets, metal foils or metal foams, areas 1B are not coveredby the electrode layer 15. These areas can be used for the subsequentbonding of an electrical terminal.

In an exemplary embodiment, a perforated aluminum expanded metal havinga thickness of 30 to 50 micrometers is initially treated with a hydrogenplasma, so that interfering surface oxide can be removed (process stepA1). In a subsequent sputter process (process step B1), a copper layeror carbon layer can be applied to this current collector, on both sidesfor example, as an intermediate layer. It is also possible, in processstep A), to dip the current collector, in the form of an aluminumexpanded metal, into a solution that etches the surface oxide and, inthe same bath or an immediately adjacent bath, to apply an intermediatelayer consisting of carbon, such as graphitic carbon. Both the copperlayer and the carbon layer, as intermediate layers, are effectivelyelectrically conductive and are not corroded by oxygen during subsequentprocess steps, which makes them corrosion-resistant. In process step B),the electrode material, in the form of activated carbon power, isinitially blade-coated as a layer with a thickness of ca. 70micrometers, together with a binder, such as polypropylene powder,polytetrafluoroethylene or polyvinyl difluoride, and a solvent, such asacetone, heptane, tetrahydrofurane or water, onto apolytetrafluoroethylene foil as the carrier foil. Following vaporizationof the solvent, these prepared layers are then rolled onto both sides ofthe current collector (laminated). The laminating rollers are heated toca. 170° C. This causes the binder to melt, so that the electrodematerial 25 is transferred to the current collector 1, forming theelectrode layer 15 and producing a homogeneous coating of the collector.In this case, the actual electrode material, the activated carbonpowder, is incorporated into a matrix consisting of binder(polypropylene), thereby forming the electrode layer.

The electrodes according to the invention and/or the method for theirproduction is not limited to the exemplary embodiment described here.Variations are possible, especially with regard to the electrodematerials used, the materials for the current collector foil, and theshapes of the perforations in the current collector foil.

1. An electrode comprising: a current collector that is two-dimensionalin shape and that contains perforations; an intermediate layer that iselectrically conductive and substantially corrosion-resistant, theintermediate layer being on at least part of a first surface of thecurrent collector, the intermediate layer comprising at least one of aprecious metal, graphitic carbon, a metal nitride and a metal carbide;and a first electrode layer that is on the intermediate layer, at leastpart of the first electrode layer being in at least some perforations ofthe current collector; the first electrode layer, the intermediate layerand the current collector being bonded together.
 2. The electrode ofclaim 1, further comprising: a second electrode layer adjacent to asecond surface of the current collector.
 3. The electrode of claim 2,wherein the first and second electrode layers comprise a same material.4. The electrode of claim 1, wherein the intermediate layer issubstantially continuous on the current collector.
 5. The electrode ofclaim 1, wherein the current collector comprises an elongated aluminumfoil.
 6. The electrode of claim 1, wherein the current collectorcomprises a net of metal wires.
 7. The electrode of claim 1, wherein thecurrent collector comprises an etched foam metal.
 8. The electrode ofclaim 1, wherein the first electrode layer comprises a conductivepolymer.
 9. The electrode of claim 1, wherein the first electrode layer,comprises activated carbon.
 10. The electrode of claim 1, wherein thefirst electrode layer comprises a metal oxide.
 11. A method forproducing an electrode, comprising: producing an intermediate layer on afirst surface of a current collector, the intermediate layer beingelectrically conductive and corrosion resistant, the current collectorbeing two-dimensional in shape and having perforations, the intermediatelayer comprising at least one of a precious metal, graphitic carbon, ametal nitride, and a metal carbide; and producing an electrode layer onthe intermediate layer; the electrode layer, the intermediate layer, andthe current collector being bonded together.
 12. The method of claim 11,further comprising producing a second electrode layer adjacent to asecond surface of the current collector.
 13. The method of claim 11,further comprising: forming the perforations by punching perforations ina metal foil; and stretching the metal foil to produce the currentcollector.
 14. The method of claim 13, wherein the metal foil comprisesaluminum; and wherein the method further comprises removing surfacelayers of the metal foil prior to producing the intermediate layer inorder to improve a conductivity of the metal foil.
 15. The method ofclaim 11, further comprising: producing the current collector by weavingmetal wires into a metal net.
 16. The method of claim 11, wherein theintermediate layer comprises a metal layer that is produced by agalvanic process, a CVD process, or a PVD process.
 17. The method ofclaim 11, wherein the intermediate layer comprises a carbon layer thatis produced by dip coating the current collector in a carbon bath. 18.The method of claim 11, wherein the electrode layer is produced by bladecoating a liquid phase containing an electrode material or a viscousphase containing the electrode material.
 19. The method of claim 18,wherein blade coating comprises: applying the liquid phase or theviscous phase to a carrier foil; drying the liquid phase or the viscousphase leaving the electrode material; and transferring the electrodematerial from the carrier foil to the current collector containing theintermediate layer.
 20. The method of claim 19, further comprisingmixing the electrode material with a binder prior to applying the liquidphase or the viscous phase to the carrier foil wherein the electrodematerial is transferred from the carrier foil to the current collectorby melting the binder.
 21. The method of claim 18, wherein the electrodematerial comprises at least one of activated carbon, a metal oxide, or aconductive polymer.
 22. An electrochemical double-layer capacitor,comprising: plural electrodes, at least one of the plural electrodescomprising: a current collector that is two-dimensional in shape andthat contains perforations; an intermediate layer that is electricallyconductive and substantially corrosion-resistant, the intermediate layerbeing on at least part of a first surface of the current collector, theintermediate layer comprising at least one of precious metal, graphiticcarbon, a metal nitride, and a metal carbide; and a first electrodelayer that is on the intermediate layer, at least part of the firstelectrode layer being in at least some perforations of the currentcollector; the first electrode layer, the intermediate layer and thecurrent collector being bonded together, the first electrode layercomprising at least one of activated carbon and graphitic carbon; and aporous separator between at least two of the plural electrodes; whereinat least one of the plural electrodes and the porous separator containan electrolyte.
 23. A hybrid capacitor, comprising: a first electrode; asecond electrode; and porous separator between the first electrode andthe second electrode; wherein the first and second electrodes and theporous separator contain an electrolyte; wherein the first electrodecomprises: a first current collector that is two-dimensional in shapeand that contains perforations; a first intermediate layer that iselectrically conductive and substantially corrosion-resistant, the firstintermediate layer being on at least part of a first surface of thefirst current collector, the first intermediate layer comprising atleast one of precious metal, graphitic carbon, a metal nitride and ametal carbide; and a first electrode layer that is on the firstintermediate layer, at least part of the first electrode layer being inat least some perforations of the first current collector; the firstelectrode layer, the first intermediate layer and the first currentcollector being bonded together, the first electrode layer comprising atleast one of active carbon and graphitic carbon; and wherein the secondelectrode comprises: a second current collector that is two-dimensionalin shape and that contains perforations; a second intermediate layerthat is electrically conductive and substantially corrosion-resistant,the second intermediate layer being on at least part of a second surfaceof the second current collector, the second intermediate layercomprising at least one of precious metal, graphitic carbon, a metalnitride, and a metal carbide; and a second electrode layer that is onthe second intermediate layer, at least part of the second electrodelayer being in at least some perforations of the second currentcollector; the second electrode layer, the second intermediate layer andthe second current collector being bonded together, the second electrodelayer comprising at least one of an electrically conductive polymer anda metal oxide.
 24. A pseudo-capacitor, comprising: plural electrodes, atleast one of the plural electrodes comprising: a current collector thatis two-dimensional in shape and that contains perforations; anintermediate layer that is electrically conductive and substantiallycorrosion-resistant, the intermediate layer being on at least part of afirst surface of the current collector, the intermediate layercomprising at least one of precious metal, graphitic carbon, a metalnitride, and a metal carbide; and a first electrode layer that is on theintermediate layer, at least part of the first electrode layer being inat least some perforations of the current collector; the first electrodelayer, the intermediate layer and the current collector being bondedtogether, the first electrode layer comprising either a metal oxide or aconductive polymer; and a porous separator between at least two of theplural electrodes; wherein at least one of the plural electrodes and theseparator contain an electrolyte.